1. Field of the Invention
The present invention relates to a power supply switching circuit for switching power supply voltages to be supplied to a load.
2. Description of the Related Art
Portable electronic devices and the like are designed to operate with a plurality of power supplies like an AC adapter and a battery. In general, a voltage of the AC adapter is higher than a voltage of the battery. When the AC adapter is connected to a rechargeable electronic device, a power supply voltage is supplied from the AC adapter to a load. In this case, a rectifier diode disposed between the power supply and the load prevents a current from flowing from the AC adapter to the battery.
However, the above-mentioned circuit may cause a power loss because the voltage supplied from the battery to the load is dropped by a forward voltage drop of the rectifier diode.
In order to solve the problem described above, a power supply switching circuit illustrated in FIG. 4 is proposed. The power supply switching circuit of FIG. 4 includes a P-type MOS transistor instead of the rectifier diode. More specifically, a P-type MOS transistor M1 is disposed between an AC adapter 23 and a load 28, and a P-type MOS transistor M2 is disposed between a battery 20 and the load 28 (see, for example, Japanese Patent Application Laid-open No. 2006-254672).
However, the conventional power supply switching circuit illustrated in FIG. 4 has a problem that if a semiconductor substrate is of P type, a higher voltage between a voltage of the AC adapter 23 and a voltage of the battery 20 may be supplied to the load 28 because of a parasitic diode existing in the P-type MOS transistor.